Stiffening card



FIG. 1 is a left side elevation view of a stiffening card, showing my new design, the right side elevation view being a mirror image of FIG. 1;

FIG. 2 is a top plan view thereof;

FIG. 3 is a front elevation view thereof; and,

FIG. 4 is a bottom plan view thereof. 

CLAIM The ornamental design for a stiffening card, as shown and described. 